Astigmatism in the condenser lenses distorts the beam to
an elliptical shape either side of focus and prevents the
beam being fully focused. It is corrected by applying two
orthogonal correction fields in the x and y directions.
Try correcting the astigmatism in the simulation opposite.
1. When both stigmator adjustments
are correct, the beam will open and close as a circle
when you adjust the C2 control. While astigmatism is still
present the beam will be elliptical at the extremes of the
C2 control.
2. Concentrate first on one of the stigmators until the
beam shape is better (less elliptical). Then adjust the
other stigmator to make it better still. You may then need
to make a small final adjustment to x and y stigmators to
complete the correction.
Why should astigmatism in the illumination system matter?
